The present invention relates to an electric current switch circuit that switches ON or OFF the electric current in response to an output of an injection bipolar logic circuit (hereinafter referred to as an IIL (Integrated Injection Logic) logic current and relates to a digital to analog converter (hereinafter referred to as a D/A converter) for which such an electric current switch circuit is suitable.
In an IIL logic circuit that realizes large scale integration and low power consumption, the withstand voltage of the IIL transistor to be sued is low. When connecting the IIL logic circuit with an external circuit, no more than about 1.5 voltage, the usual withstand voltage of the IIL transistor, should be applied to the output of the IIL logic circuit.
FIG. 7 is an electric circuit diagram showing a structure of a D/A converter 1 that is realized by a plurality of typical conventional electric current switch circuits which receive the outputs of the respective IIL logic circuits. The D/A converter 1 is an N-bit R-2R resistor ladder type. The D/A converter 1 is composed of electric current switch circuits sw1, sw2, . . . , and swN. A line 2 to which a reference voltage Vref is applied is commonly connected with one end of resistors r11, r12, . . . , and r1N in the respective electric current switch circuits sw1, sw2, . . . and swN. Resistors r22, r23, . . . , and r2N are connected between the other ends of adjacent resistors r11, r12, . . . , and r1N. Such other ends are electrically switched ON and/or OFF with respect to a ground line by the respective electric current switch circuits sw1, sw2, . . . , and swN. The resistors r11, r12, . . . , and r1N and the resistors r22 r23, . . . and r2N constitute a resistor ladder circuit. The reference voltage Vref is divided in accordance with the combinations of the resistors r11, r12, . . . , and r1N and the resistors r22, r23, . . . ,and r2N so as to be outputted to an output terminal 3 as a D/A converted signal from the above-mentioned other end of the resistor r1N in the electric current switch circuit swN of the last stage.
In the electric current switch circuit sw1, transistors of NPN type (hereinafter referred to as NPN transistors) q1 and q2 form a pair. The emitters of the NPN transistors q1 and q2 are commonly connected with the collector of a NPN transistor q3, and its emitter is connected with the ground through a resistor r1. In the NPN transistor q1, a constant voltage of 1.4V is applied to the base, and the collector is connected with a point connecting the resistor r11 with the resistor r22. In the NPN transistor q2, the collector is connected with the line 2, the base is connected with a point connecting transistors of PNP type (hereinafter referred to as PNP transistors) q4 and q5 that are provided so as to be connected in series with each other between the ground and a power source line 4 to which a high level Vcc is applied. A voltage of 1.4V is applied to the base of the PNP transistor q5. The point connecting PNP transistors q4 with q5, i.e., the base of the NPN transistor q2 is connected with the collector of an NPN transistor q6. The base of the NPN transistor q6 receives an output of the IIL logic circuit c1, and is connected with the power source line 4 to which the high level Vcc is applied. The emitter of the NPN transistor q6 is connected with the ground line, i.e., is grounded.
The D/A converter 1 is further provided with a constant current generation circuit 5 in which a constant electric current obtained by multiplying constant current Iref by a predetermined numeral value. The constant electric current is generated by (1) a pair of NPN transistors q11 and q12 that constitute a current mirror circuit, (2) a NPN transistor q13, (3) resistors r31 through r33, and (4) a constant current source f. The constant electric current flows to the ground through the NPN transistor q3 that constitute a current mirror circuit together with the NPN transistors q11 and q12. The constant electric current flows to the ground through an NPN transistor q14 that constitute a current mirror circuit together with the NPN transistors q11 and q12. This allows the constant electric current to flow through a PNP transistor q15 that is connected in series with the NPN transistor q14. Accordingly, the constant electric current flows through the respective PNP transistors q4 and q7 that constitute a current mirror circuit together with the PNP transistors q15.
With the electric current switch circuit sw1 having the above structure, the current that flows to the ground through the NPN transistor q3 is switched ON or OFF in accordance with the difference between the base voltages of the NPN transistors q1 and q2. More specifically, when the base voltage of the NPN transistor q1 is fully greater than that of the NPN transistor q2, the electric current flows to the NPN transistor q3 from the resistor ladder circuit. In contrast, when the base voltage of the NPN transistor q1 is fully smaller than that of the NPN transistor q2, the electric current does not flow to the NPN transistor q3 from the resistor ladder circuit. The electric current switch circuits sw1 through swN are connected in series with each other in accordance with the resolution required for the D/A converter. Thus, the D/A converter 1 of an N-bit R-2R resistor ladder type is constituted. Note that the electric current switch circuits sw2 through swN have the same structure as the electric current switch circuits swl. The electric current switch circuits sw2 through swN switch ON and/or OFF in response to the outputs of the respective IIL logic circuits c2 through cN.
In order not to have the NPN transistor q3 saturated so as to maintain the constant electric current, it is always necessary to apply a voltage of not less than (Vr1+Vdsat) to the collector of the NPN transistor q3. Note that the voltage Vr1 indicates the drop voltage across the resistor r1, and the voltage Vdsat indicates the saturated voltage of the NPN transistor q3. The. saturated voltage Vdsat falls within the range of about 0.1V and 0.3V. When the drop voltage Vr1 is equal to 0.3V, it is necessary to apply a voltage of not less than 0.6V to the collector of the NPN transistor q3. The drop voltage Vbe between the base-emitter of the transistor falls within the range of about 0.6V and 0.8V. Accordingly, it is necessary to satisfy the following requirement, i.e., it is necessary to apply a voltage of not less than 1.4V (=Vr1+Vdsat+Vbe) to at least one of the bases of the NPN transistors q1 and q2. According to the electric current switch circuit sw1, in order to satisfy the above requirement, a constant voltage of 1.4V is applied to the base of the NPN transistor q1, as has been described above, so as to carry out the switching operation by changing the base voltage of the NPN transistor q2.
In the mean time, when considering the difference between the base voltages of the NPN transistors q1 and q2 required for the switching, in order to make the collector current ratio of the NPN transistors q1 and q2 be 1:1000 so as to fully carry out the switching, it is necessary to maintain the voltage difference of (VTxln(1000)) between the base voltages of the NPN transistors q1 and q2. Note that VT indicates a thermal voltage, and has a voltage of about 26 mV when the element temperature is at 25xc2x0 C. while 37 mV when the element temperature is at 150xc2x0 C. A voltage of not less than 256 mV is required for the voltage difference between the base voltages of the NPN transistors q1 and q2, when it is assumed that VT is equal to 37 mV. Thus, in order to fully carry out the switching, it is necessary for the base voltage of the NPN transistor q2 to be a voltage of not less than 1.66V or a voltage of not more than 1.14V which results from 1.4Vxc2x1256 mV.
When directly connecting the bases of the NPN transistors q2 with the outputs of the IIL logic circuits c1 through cN, respectively, it is likely that a voltage of not less than 1.66V is applied to the respective IIL logic circuits c1 through cN. This means that a voltage greater than the withstand voltage of the respective IIL logic circuits c1 through cN is often applied so as to cause lower reliability or this deficiency, a level change circuit is needed by which the voltage applied to the output off the IIL logic circuit c1 is suppressed to be not more than the withstand voltage of the IIL transistor and the base voltage of the NPN transistor q2 is suppressed to a voltage of not less than 1.66V and of not more than 1.14V. In the electric current switch circuits sw1 through swN, such a level change circuit is composed of the PNP transistors q4, q5, and q7 and the NPN transistors q6.
More specifically, each output of the IIL logic circuits c1 through cN is connected with the base of the NPN transistor q6 to which a bias electric current is applied by the PNP transistor q7. The PNP transistor q4 supplies the base of the NPN transistor q2 with a bias electric current. The collector of the PNP transistor q4 is connected with the emitter of the PNP transistor q5 whose collector is connected with the ground so that the PNP transistor q4 has not saturated.
When the outputs of the respective IIL logic circuits c1 through cN are in an ON state, each bias electric currents applied by the PNP transistor q7 flows into each of the respective IIL logic circuits c1 through cN. This causes the base voltage of the NPN transistor q6 to be substantially zero so as to switch off the NPN transistor q6. As a result, the base voltage of the NPN transistor q2 increases and the PNP transistor q5 becomes in operation, thereby resulting in that the base voltage of the NPN transistor q2 becomes (1.4V+Vbe), i.e., falls within the range of 2.0V and 2.2V.
In contrast, when the outputs of the respective IIL logic circuits c1 through cN are in an OFF state, each bias electric current applied by the PNP transistor q7 flows not into each of the IIL logic circuits c1 through cN but into the base of the NPN transistor q6. This causes the base voltage of the NPN transistor q6 to be Vbe, i.e., to fall within a range of 0.6V and 0.8V so as to switch ON the NPN transistor q6. As a result, the base voltage of the NPN transistor q2 decreases to substantially zero.
The base voltage of the NPN transistor q2 receives a voltage of a range of 2.0V and 2.2V or a voltage of about zero, thus the switching operation of the electric current is carried out. However, as has been described above, the voltage of not more than 0.8V is applied to each output of the IIL logic circuits c1 through cN so that a voltage of more than the withstand voltage of the IIL transistor is not applied.
According to the foregoing convention art, seven transistors were required (q1 through q7) and one resistor r1 for each of the switch circuits sw1 through swN including the level change circuit. Accordingly, the D/A converter 1 having such a plurality of electric current switch circuits occupies a large mounting area on an IC chip. This causes the problems that it is impossible to mount the D/A converter 1 on an IC chip of a compact package and the manufacturing cost increases.
In the case where an electric current switch is in an OFF state, i.e., the base voltage of the NPN transistor q2 is greater than that of the NPN transistor q1, the electric current flowing through the collector of the NPN transistor q2 is not necessary for the operation of the D/A converter 1. This raises the problem of unnecessary power consumption.
Further, the following problems also arise. It becomes necessary to apply a voltage of at least 1.4V to the base of the NPN transistor q1 because of the foregoing reason. It is not possible to make the collector voltage of the NPN transistor q1 be not more than 0.9V (=Vr1+Vdsat(q3)+Vdsat(q1)) so as not to have the NPN transistor q1 saturated. This restricts applicability of the circuit in that the D/A converter 1 is made to be operated at a low power source voltage.
It is an object of the present invention to provide an electric current switch circuit that is capable of reducing the lower limit of the output voltage with small circuit scale and low power consumption, and also relates to provide a D/A converter using such an electric current switch circuit.
In order to achieve the foregoing object, an electric current switch circuit in accordance with the present invention which switches ON and/or OFF the electric current in response to an output of an IIL logic circuit is characterized by including (a) a first transistor of NPN type, that has a base connected with an output terminal of the IIL logic circuit and a collector that is an output terminal through which an electric current flows, for switching the electric current, (b) a first resistor through which an emitter of the first transistor is grounded, (c) a first constant current source for supplying the base of the first transistor with a bias electric current, (d) a second transistor of PNP type, that has an emitter connected with the base of the first transistor and a collector being grounded, for controlling (limiting) a voltage of the output terminal of the IIL logic circuit, and (d) a voltage bias circuit for supplying the base of the second transistor with a bias voltage.
With this arrangement, when the output of the ILL logic circuit is in an OFF state, the base of the first transistor receives the bias voltage that is higher by the voltage Vbe of the second transistor than the bias voltage outputted from the voltage bias circuit. The emitter of the first transistor receives the voltage which is lower by the voltage Vbe of the first transistor than the base voltage of the first transistor. In other words, the emitter of the first transistor receives substantially the same voltage as the above-mentioned bias voltage. As a result, the first transistor switches ON so that a constant electric current flows through the collector toward the output terminal. When the drop voltage VR1 across the first resistor is 0.3V, for example, only a voltage of 0.9V to 1.1V (=VR1+Vbe) is applied to the base of the first transistor that is connected with the output terminal of the IIL logic circuit, even when the IIL transistor is in an OFF state. Accordingly, a voltage more than the withstand voltage of the IIL transistor is not applied to the base of the first transistor.
In contrast, when the IIL transistor is in an ON state, the base voltage of the first transistor substantially becomes zero voltage. The first transistor switches OFF so as to make substantially zero the electric current flowing through the output terminal. Thus, it is possible to realize an electric current switch circuit that can carry out the switching in accordance with the output of the IIL logic circuit, similar to the case where the conventional electric current switch circuit and the
When it is assumed that the first constant current source is realized by a single PNP transistor, the electric current switch circuit can be realized by three transistors and one resistor according to the electric current switch circuit of the present invention. This allows the chip area to be reduced. Especially, in a circuit which includes a plurality of the foregoing electric current switch circuits, like a D/A converter, it is possible to mount them on an IC in a compact package and is possible to avoid an increase in manufacturing cost.
Unlike the conventional electric current switch circuit, since there are no structures for generating and consuming the constant electric current, other than the first constant current source for driving the first transistor, it is possible to reduce power consumption, irrespective of the switching ON or OFF of the first transistor.
Further, it is possible to lower the voltage of the output terminal for electric current, i.e., the collector voltage of the first transistor up to 0.6V (=VR1+Vsat). Therefore, it is possible to relieve the restriction required for operating the D/A converter or other devices having the foregoing electric current switch circuits in a lower source voltage. It is possible to lower the lower limit of the output voltage, accordingly.
According to an exemplary embodiment of the electric current switch circuit of the present invention, the foregoing voltage bias circuit is provided with a feedback circuit in which an input is applied through an input terminal for the electric current and the bias voltage is sent to the base of the second transistor through an output, a third transistor having a collector that serves as the input terminal for the electric current, a second resistor through which an emitter of the third transistor is grounded, a second constant current source for supplying a base of the third transistor with a bias electric current, and a fourth transistor having an emitter connected with the base of the third transistor, a collector grounded, and a base connected with the output terminal of the feedback circuit.
With this arrangement, the output voltage of the feedback circuit is controlled so that the electric current, which flowed to the input terminal for the electric current, flows to the collector of the third transistor.
Accordingly, when the output of the IIL logic circuit is in an OFF state, the same electric current which flowed to the input terminal for the electric current flows through the output terminal. Further, when the circuit elements constituting the electric current switch circuit have the same characteristics as those of the circuit elements constituting the voltage bias circuit, the above-mentioned electric current is not affected by the changes of the characteristics of the circuit element, the ambient temperatures, or other factors.
According to another exemplary embodiment of the electric current switch circuit of the present invention, the feedback circuit is constituted by a fifth transistor of NPN type having a collector to which a power source voltage is applied, an emitter grounded through a third resistor that serves as the output of the feedback circuit, and a base that serves as the input of the feedback circuit.
With this arrangement, it is possible to realize the feedback circuit by an extremely simple circuit configuration.
According to another exemplary embodiment of the electric current switch circuit of the present invention, the feedback circuit is constituted by an operational amplifier. More specifically, the input of the feedback circuit is sent to a non-inverting terminal of the operational amplifier, a reference voltage of a reference voltage source is sent to an inverting input terminal of the operational amplifier, and an output terminal of the operational amplifier serves as the output of the feedback circuit.
With this arrangement, it is possible to obtain from the output terminal the output electric current that has the same value as the electric current that flowed through the input terminal for the electric current while the voltage of the input terminal for the electric current is maintained to have the same value as the reference voltage.
According to another exemplary embodiment of the electric current switch circuit of the present invention, the collector of the fifth transistor is connected with a collector of a sixth transistor of PNP type that serves as an input of a current mirror circuit, and seventh and eighth transistors of PNP type that serve as the respective first and second constant current sources as well as serve as respective outputs of the current mirror circuit.
With this arrangement, it is possible to obtain the reference electric currents outputted from the respective first and second constant current sources through the collector of the fifth transistor.
Accordingly, it is not necessary to newly provide current sources for generating the reference electric currents. It is possible to simplify the circuit configuration and to save power consumption.
A D/A converter in accordance with an exemplary embodiment of the present invention is provided with a plurality of electric current switch circuits having the structure of any one of the above-mentioned ones that are connected in series with each other in which the bases of the respective second transistors are commonly connected with each other.
With this arrangement, it is possible to obtain many electric current switch outputs that can be independently switched ON and/or OFF and that have a single electric current value. Thus, the D/A converter having many electric current switch circuits can be mounted on an IC in a compact package. Further, it is possible to avoid increased manufacturing costs.
The scope of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of present invention. illustration only, and thus, are not limitative of the present invention.